In a variety of environments, such as in process control systems, analog or digital signals must be transmitted between diverse sources and circuitry using those signals, while maintaining electrical (i.e., galvanic) isolation between the sources and the using circuitry. Isolation may be needed, for example, between analog sensors and amplifiers or other circuits which process their output, or between microcontrollers, on the one hand, and sensors or transducers which generate or use microcontroller input or output signals, on the other hand. Electrical isolation is intended, inter alia, to prevent extraneous transient signals, including common-mode transients, from inadvertently being processed as status or control information, or to protect equipment from shock hazards or to permit the equipment on each side of an isolation barrier to be operated at a different supply voltage, among other known objectives. One well-known method for achieving such isolation is to use optical isolators that convert input electrical signals to light levels or pulses generated by light emitting diodes (LEDs), and then to receive and convert the light signals back into electrical signals. Optical isolators present certain limitations, however: among other limitations, they are rather non-linear and not suitable for accurate linear applications, they require significant space on a card or circuit board, they draw a large current, they do not operate well at high frequencies, and they are very inefficient. They also provide somewhat limited levels of isolation. To achieve greater isolation, opto-electronic isolators have been made with some attempts at providing an electrostatic shield between the optical transmitter and the optical receiver. However, a conductive shield which provides a significant degree of isolation is not sufficiently transparent for use in this application.
One isolation amplifier avoiding the use of such optical couplers in a digital signaling environment is described in U.S. Pat. No. 4,748,419 to Somerville. In that patent, an input data signal is differentiated to create a pair of differential signals that are each transmitted across high voltage capacitors to create differentiated spike signals for the differential input pair. Circuitry on the other side of the capacitive barrier has a differential amplifier, a pair of converters for comparing the amplified signal against high and low thresholds, and a set/reset flip-flop to restore the spikes created by the capacitors into a logic signal. In such a capacitively-coupled device, however, during a common mode transient event, the capacitors couple high, common-mode energy into the receiving circuit. As the rate of voltage change increases in that common-mode event, the current injected into the receiver increases. This current potentially can damage the receiving circuit and can trigger a faulty detection. Such capacitively coupled circuitry thus couples signals that should be rejected. The patent also mentions, without elaboration, that a transformer with a short R/L time constant can provide an isolation barrier, but such a differential approach is nonetheless undesirable because any mismatch in the non-magnetic (i.e., capacitive) coupling of the windings would cause a common-mode signal to appear as a difference signal.
Another logic isolator which avoids use of optical coupling is shown in commonly-owned, unpublished U.S. patent application Ser. No. 08/805,075, filed Feb. 21, 1997, in the name of Geoffrey T. Haigh, titled "Logic Isolator with High Transient Immunity," incorporated by reference herein. This logic isolator exhibits high transient immunity, for isolating digital logic signals, such as signals between equipment on a field side (i.e., interfacing with physical elements which measure or control processes) and microcontrollers on a system control side, useful in, for example, a process control system. In one aspect, the logic isolator has an input circuit that receives a digital input signal, with edge detection circuitry that detects rising and falling edges of that input signal. The logic circuit provides an output signal indicative of those rising and falling edges to a transformer assembly which serves as an isolation barrier. The transformer assembly replicates the signal it receives and provides it to an output circuit, while shunting capacitive common-mode transient currents to ground. The output circuit converts the signal from the transformer back into a digital logic signal with rising and falling edges as in the digital input signal, those slightly delayed therefrom. The transformer assembly preferably includes a link-coupled transformer that has a first core with a first winding, a second core with a second winding, and a grounded link wire that extends from the first core to the second core for grounding capacitively-linked common-mode transients. Alternatively, a shielded transformer, with a grounded double- or single-shield between primary and secondary windings, could be used.
In certain described embodiments, the input circuit converts the rising and falling edges in the digital input signal to positive and negative pulses using tri-level logic, and the output circuit converts these pulses back into rising and falling edges. The input circuit preferably also includes a pulse generator for providing pulses, referred to as refresh pulses, with a high frequency and with a pulse width that is the same as the width of the pulses created in response to detection of a rising edge or a falling edge. The refresh pulses are logically combined with the input signal to provide an interrogating functionality that allows the isolator to determine the DC- or steady- state of the input signal; therefore, the isolator can recover quickly in case of a power spike or dropout, and also can quickly determine the state if an edge is missed. The isolator has circuitry that inhibits the first refresh pulse after an edge to prevent a double-wide pulse being transmitted; consequently, the isolator can interrogate the state of the input signal a time t after an event, with T&lt;t&lt;2T--i.e., no later than 10 .mu.s if the refresh pulses have a period of 5 .mu.s.
A need, however, exists for an isolation technology which is useful for both analog and digital signals. A further need exists for isolators which are manufacturable with still lower cost than the aforementioned types of isolators, operating at lower power and manufacturable in very small size. Even more specifically, a need exists for isolators employing magneto-resistive (MR) and giant magneto-resistive (GMR) effects and which are very fast in operation (being useful in the nanosecond domain).